AVIFormat: inject contexts into codecs

Most of the codecs used in the AVIFormat were not receiving a Context.
Now, they do.
AVIFormat: fix codec use

There were two mistakes in how Codecs were being used in the AVIFormat:
* positions were only seeking properly when called from openPlane
* No paranthesees when computing number of channels!

With these issues fixed, the AVIFormat should now decompress via codec
appropriately.

Closes #228
Bump to next development cycle

Signed-off-by: Jenkins <jenkins@dev.loci.wisc.edu>
ReaderTest: add reader reuse tests

These unit tests test reader behavior when calling setSource or
setMetadata on an existing Reader instance.
WIP: fix reader reuse.

TODO: DimensionSwapper, FileStitcher, MinMaxFilter all suck still.
WIP: need imagemetadata generation to be lazy
WIP